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542

Let ABCDEFGH be the plan of the arch; IKLM the outer line; and NOPQ the inner line on the elevation.

abcde, on the elevation, shows the bevel of each joint or bed from the face of the wall; and a b c d e below, gives the mould for the same, y on the elevation corresponds with x y at a. where

The arch mould, fig. 551, No. 2, is applied on the face of the stone, and on being applied to the parts of the plan, gives, of course, the beve of each concave side of the stone with the face, that is K to O, on the elevation.

Fig. 552. To find the mould for constructing a semicircular arch in a circular wall.

No. 1 is the elevation of the arch; and No. 2 the plan of the bottom bed from q to r.

a to b is what the arch gains on the circle from the bottom bed ko tol; and c to d is the projection of the intrados to p, on the joint l. p.

Nos. 2, 3, 4, are plans of the three arch-stones, 1, 2, 3, in the elevation; and Nos. 5 and 6 are moulds to be applied to the beds of stones 1 and 2, in which s c equals s c in No. 2, and tw equals tw in No. 3. In No. 1, klpo is the arch or face mould.

When the reader is thoroughly proficient in the construction of arches, under given datas, as the circumstances of the case may point out, he may proceed to investigate the principles of spherical domes and groins.

Figs. 553 and 554 show the principles of developing the soffits of the arches in the two preceding examples. In each the letters of reference are alike, and the operation is precisely the same.

Let ABDE be the plan of the opening in the wall; and AFB the elevation of the arch: produce the chord AB to C, divide the semicircle AFB into any number of parts, the more the better, and with the compasses set to any one of these divisions, run it as many times along AC as the semicircle is divided into; then draw lines, perpendicular to BC, through every division in the semicircle and the line CA, and set the distance 1b, 2 d, 3 f, &c. respectively equal to a b, c d, ef, &c. and then by tracing a curve through these points, and finding the points in the linc GD, in the same manner, the soffit of the arch is complete.

Fig. 555, shows the method of constructing spherical domes.

No. 1 mould is applied on the spherical surface to the vertical joints; and No. 2 mould on the same surface to the other joints; and in both cases, the mould tends to the centre of the dome.

3, 4, 5, 6, 7, and 8, are moulds which apply on the convex surface to the horizontal joint, the lines ab, c d, ef, &c. being at right angles to the different radii, b c, d c. fc, &c. and produced until they intersect the perpendicular ac; the different intersections are the centres which give the circular leg of the mould, and the straight part gives the horizontal joint.

Fig. 556 exhibits the plan of a groined vault.

Lay down the arch, either at the full or half size, on a floor or piece of floor-cloth, then divide and draw on the plan the number of joints in the semicircular arch, and from the intersections with the diagonals, draw the transverse joints on the plan, and produce them till they touch the intradoes of the elliptical arch, the curve of which may be found by setting the corresponding distances from the line of the base to the curve; thus ab equal to a b. This being accomplished, draw the joints of the ellipti

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cal arch in the manner of which we give c d, as a specimen. To draw the joint cd, draw the chord e c and bisect it, draw a line from the centre c, through the bisecting point, and produce it till it touches the perpendicu lar ef; and cd, being at right angles to ef, will be the joint required. In the same manner the others are found.

By examination, it will be seen, that a rectangle circumscribing the mould 3, 3, gives the size of the stone in its square state, and, that if each stone in both arches be thus enclosed, the dimensions for each will be found, as also the position in which the moulds must be placed. The dark lines give the different bevels which must be carefully prepared and applied to the stones in the manner represented in the figure.

Fig. 557. To draw the joints of the stones for an elliptical arch in a wall, &c.

The curve is here described by the intersection of lines, which, certainly, gives the most easy and pleasing curve, as segments of circles apply only under certain data, or in the proportion which the axis major has to the axis minor, while the intersection of lines apply to any description of ellipsis. Find the foci F. In an ellipsis the distance of either focus from one extremity of the axis minor is equal to the semi-axis major; that is, DF is equal to c C. Then to find any joint, a b, draw lines from both foci through the point b, as Fe, fd, and bisect the angle db e by the line ab, which is the joint required.

Having thus given a general outline of the principles of masonry, and accompanied the same with a few examples on the most abstruse parts of the art, we shall conclude this part of our treatise with the methods employed in the mensuration of masons' work.

Rough stone or marble is measured by the foot cube: but in measuring for workmanship, the superficies or surface, for plain work, is measured before it is sunk. In measuring ashlar, one bed and one upright joint are taken and considered plain work. In taking the plain sunk, or circular work, and the straight moulded, or circular moulded work, particular care is required to distinguish the different kinds of work in the progress of preparing the stone. In measuring strings, the weathering is denominated sunk work, and the grooving throatings.

Stone cills to windows, &c. are, in general, about 4 inches thick and 8 inches broad, and are weathered at the top, which reduces the front edge to about 4 inches, and the horizontal surface at the top to about 1 inch on the inside; so that the part taken away is 6 inches broad and three quarters of an inch deep. Cills, when placed in the wall, generally project about 24 inches. The horizontal part left on the inside of the cill is denominated plain work; and the sloping part sunk work; and in the dimension book are entered thus,

2

4

11 4

21

8 inches the breadth of the plain work in the cill according to the above dimensions, then

8 28 Plain work.

62 2 Sunk work.

4 6 Plain to ends.

of throating.

No account is taken of the sawing.

Cornices are measured by girthing round the moulded Farts, that is, the whole of the vertical and under parts, called moulded work :-for example, suppose a cornice project one foot, girth two feet, and is 40 feet in length, then the dimensions will be entered as under,

[blocks in formation]

All the vertical joints must be added to the above.

Cylindrical work is measured in the girth; and the surface is calculated to be equivalent to plain work twice taken.

For example, suppose it be required to measure the plain work or a cylinder, 10 feet long, and 5 feet in circumference, the dimensions would then be entered

10 0 500 Sup'. plain work, double measure.

5 0

Paving-slabs and chimney-pieces are found by superficia measure, as also are stones under two inches thick.

The manner in which the dimensions of a house are taken, vary according to the place and the nature of the agreement.

In Scotland, and most parts of England, if the builder engages only for workmanship, the dimensions are taken round the outside of the house for the length, and the height is taken for the width, and the two multipiiea

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